This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 奲 is uncertain. It visually combines 奢 and 單, but the original semantic and phonetic roles are unclear.
Since the meaning is unclear, focus on the shape: 奢 (extravagance) on the left and 單 (single) on the right. Imagine a single extravagant item that is so large it fills the whole character.
For シャ, picture a shah (a king) admiring a single extravagant treasure: shah -> シャ, and the treasure is so grand it is the only one.